The present invention relates to an apparatus for automatically determining bulk specific gravities of powdery products such as resin powder of, for instance, vinyl chloride resins, ABS resins and MBS resins; powder of foods and cement.
When preparing resin powder or forwarding the powdery product from a manufacturing plant, the product is in general subjected to analysis or quality inspection for various predetermined properties. The results thus obtained are sent back to the manufacturing plant for the improvement of production processes or they are used in the denotation of the quality in order to afford convenience to the destination or the consignee.
In case of, for instance, vinyl chloride resin powder, one of the items for quality inspection thereof is to determine the bulk specific gravity. The determination of bulk specific gravity is in general performed using a funnel provided with a damper and a cylindrical receiver as defined by JIS-K-6721. According to JIS-K-6721, a powdery sample sufficiently admixed is introduced into the funnel and immediately thereafter, the damper is pulled out so that the powdery sample drops into the receiver. After the excess powdery sample is scraped off, by a glass bar, from the receiver which is heaped with the powdery sample, the weight of the receiver filled with the powdery sample (the gross weight) is determined accurately. The weight of the receiver per se (the tare weight) is subtracted from the gross weight to give the true weight of the powdery sample (the net weight) and then the net weight of the sample is divided by the inner volume of the receiver to thus obtain the bulk specific gravity of the sample.
Reproducible results cannot always be obtained even if a specific powdery sample is inspected for the bulk specific gravity several times, while faithfully following the regulation defined by JIS-K-6721. In particular, the measured values for a particular powdery sample widely vary depending on operators. The inventors of this invention have conducted various investigations on this point and have found out that such scattering in the measured values is caused due to subtle differences in the manner of measuring operations for every measurement. In particular, the manner of operations significantly vary depending on operators and this results in the scattering in measured values. For instance, the packing density of a powdery sample observed when the sample is quickly introduced into the funnel substantially differs from that observed when it is gently introduced into the funnel. This is delicately reflected in the velocity of the powdery sample dropped through the funnel and in turn in the packing density of the powdery sample in the receiver.
In addition, the density of the powdery sample in the receiver is influenced by the difference in the speeds of the damper pulled out from the funnel and the contact between the damper and the powdery sample filled to the brim upon dropping the sample into the receiver and the removal of the heaped-up powdery sample by scraping it off with the glass bar. Moreover, it has also been elucidated that the charging of the powdery resin sample during, for instance, transmission through a piping and any change in the environment for measurement greatly affect the measured values of the bulk specific gravity thereof. For instance, the humidity in the environment enclosing the measuring system is greatly influenced by the weather change and this is reflected in the rate of electrification of the resin powder and hence the packing density of the powdery sample in the receiver. Accordingly, reproducible results cannot be obtained.
Furthermore, in the conventional apparatus for determining the bulk specific gravity, such operations must be manually carried out. Therefore, the conventional apparatus requires much labor and time and correspondingly the efficiency thereof becomes low. In particular, the receiver filled with the powdery sample should be handled with the greatest circumspection. Besides, the receiver must be completely cleaned after each measurement is completed to ensure the precision of the measurement subsequently performed. This makes the measurement quite troublesome.